The present invention relates to a radio frequency (RF) coil system for magnetic resonance imaging (MRI), and more particularly to the RF coil system having a transmission line, combined by a distributed constant line and a lumped constant circuit, which connects an RF coil to an interface unit.
In MRI, one of the prior arts offers an imaging apparatus which has an RF coil system including a receiving probe for intra-cavities. The receiving probe is inserted into patient body cavities such as a rectum and receives a magnetic resonance (MR) signal for acquiring reconstructed images of the cavity wall and its wall-inner portion. The MR signal is provided by excitation of nuclei with an RF signal transmitted from, for example, by a whole-body coil as a separate transmitting RF coil set around the patient.
The receiving probe is exemplified by `A Disposable Prostate Probe and Interface System For High Field`, George J. Misic et al. SMRM 8th Annual Meeting 1989. Page 179', for instance. In this probe and interface system, a receiving coil is connected through a coaxial cable to a PIN diode in an interface box. The whole length of the coaxial cable determined to be a length of .lambda./4 when .lambda. is an wavelength of an MR signal. Thus, by utilizing the impedance conversion effect of the one quarter wavelength, the receiving coil is decoupled from an excitation magnetic field during the transmission phase.
There is provided another RF coil system using crossed diodes. The RF coil system is only for receiving an MR signal and comprises a single loop coil, a coaxial cable serving as a distributed constant line connected to the loop coil, and an interface unit connected between the coaxial cable and an image processor. The whole length of the coaxial cable is determined to be a length of .lambda./4 (.lambda. is an wavelength of an MR signal). A resonance capacitor is interleaved in the loop coil for forming a resonance circuit by the loop coil itself and capacitor.
The loop coil and coaxial cable form a part of probe, the part being inserted into body cavities. The interface unit incorporates crossed-diodes, a tuning capacitor, and a pre-amplifier, which are placed from the coaxial cable side, in turn.
Owing to a standing wave arising along the coaxial cable, the above-said effect of the one quarter wavelength (that is, the impedance conversion effect between one end and the other end of the cable) can also be utilized in order to decouple the loop coil from an RF magnetic field for excitation produced by a separate transmitting RF coil. As a result, no current is induced through the loop coil, and there will be almost no disturbance in the excitation RF field.
However, it is pointed out that the coaxial cable becomes rather long; for example, supposing that an wavelength contraction rate is approximately 0.68 for coaxial cables normally used and a static magnetic field of 0.5 tesla is accomplished (in this case, a resonant frequency of objective protons is 21.3 [MHz]), .lambda./4=2.4 [m]. The lower the strength of the magnetic, the lower the Larmor frequency. This means that the coaxial cable becomes further longer for maintaining the length of .lambda./4 .
Such RF coil systems having the long coaxial cable cause different drawbacks as follows. First, for preparation and insertion into body cavities, the receiving probe (the part of the coil itself and the coaxial cable) gives an operator troublesome handling. This results in reduced maneuverability, thus reducing the throughput of diagnosis because of long operation time. At the same time, such receiving probes are undesirable for storage.
Second, longer coaxial cables cause greater signal loss and there will be a possibility of reducing a ratio of signal to noise. Third, it is required that the receiving probe, inserted into body cavities, should be disposable in view of sanitary affairs. Hence the use of longer coaxial cables give rise to higher manufacturing cost of the probe, and inspection cost per one time of diagnosis becomes also higher.